Lymphocytes from lymph nodes obtained at mastectomy in breast cancer patients have been fused with murine non-Ig producer myeloma cells to obtain human-mouse hybridoma cultures that synthesize human monoclonal antibodies. Immunoglobulin (Ig) production was stable in many of the cloned cultures through a 60-300 day observation period, with levels of human Ig synthesis ranging from 0.1-20ug/ml of supernatant fluid. Using the immunoperoxidase technique and tissue sections, several human IgM's demonstrated binding to human mammary tumor cells. One human IgM monoclonal antibody was used to discriminate between mammary carcinoma cells (from 55 of 59 patients) and normal mammary epithelial cells, stroma, or lymphocytes of the same breast. These studies demonstrate that stable clones of human-mouse hybridomas, secreting human Ig, can be generated using lymph nodes of mastectomy patients.